Sealing and inhibiting corrosion of anodized aluminum



United States Patent 3,257,244 SEALING AND INHIBITING CORROSION OFANODIZED ALUMINUM Erik Fredrik Barkman, Henrico County, Va., assignor toReynolds Metals Company, Richmond, Va., 21 corporation of Delaware NoDrawing. Filed Oct. 14, 1964, Ser. No. 403,944 18 Claims. (Cl. 148--6.2)

This application is a continuation-in-part of my copending applicationSerial. No. 59,509, filed September 30, 1960, now abandoned.

This invention relates to an improved method of aquelayers on aluminumand aluminum base alloys by the application of corrosion inhibiting andsealing agents to such layers. More particularly, the invention concernsthe treatment of anodically formed aluminum oxide layers of films withwater soluble basic organic nitrogen compounds, alone or in combinationwith one or more water soluble inorganic salts of metals of Group VI ofthe Periodic -System. Another aspect of the invention concerns thetreatment of anodically formed aluminum oxide layers with combinations.of said Water soluble inorganic salts per se. 1

Aluminum oxide layers are conventionally formed by anodizing proceduresinvolving the use of baths containing sulfuric acid, aliphaticandalicyclic water soluble carboxylic acids, chromic acid, or phosphoricacid, or mixturesor combinations of these acids to provide anelectrolyte of the oxide dissolving type. The anodizing is customarilyaccomplished by the passage through the electrolyte in which thealuminum is the anode, of direct or alternating current, or acombination thereof. A typical anodizing bath is one containing fromabout to 20 percent of sulfuric acid by weight, at a temperature of68-72 F., using direct current at a voltage of'about 5 to 20 volts at acurrent density of 10-15 amperes per square foot. The duration of thetreatment determines the thicknes of anodic film formed.

The novel method of the present invention is adapted for retardingcorrosive action on aluminum oxide films of all types, including brightanodized, matte surface, and hard-coat anodic coatings, but it isespecially valuable in conjunction with bright anodized surfaces.

v The principal purpose of an anodic film is that of providing amechanical and chemical protective layer to prevent general or selectivecorrosion or atmospheric oxidation of the underlying aluminum metalsurface. The amount of possible corrosive action is determined by theenvironment in which the anodized article is employed. Thus forautomotive bright trim and various types of domestic appliances wherefrequent exposure to moisture, salt, dirt, and other corrosive agentsoccurs, the highest possible quality of the anodic surface is requiredto retain the initial attractive appearance. For such applicationsanodic film thicknesses in-the range of 0.1-0.6 mil are used. There arealso employed for such anodized aluminum alloy surfaces having opticalproperties which entitle them .to be classed as bright anodic surfaces,aluminum base alloys having a chemical and metallurgical compositionpromoting the formation of a relatively clear transparent anodic filmupon anodizing. Such alloys are those containing a minimum quantity ofelements other than aluminum with the exception of alloys containingmagnesium and magnesium silicide. Commercial designations of thesealloys include Aluminum Association numbers 1199 (super pure aluminum),1188, reflectance sheet 5357, 5457, 5557, 5657, 6063, and 6463. Brightanodic coatings on all-of these alloys are capable of being improved bythe novel process of the present invention.

It is well known that during anodizing in commercial ous hydration ofanodically formed aluminum oxide "ice electrolytes such as sulfuric acidwhich provides continuous solvent action on the aluminum oxide filmduring formation, pores are formed which provide the necessary channelsfor passage of current through the film during the continuing growth ofthe oxide layer. The resulting film structure consists of large numbersof hexagonally shaped oxide cells. The center of each hexagon containsthe pore, the dimensions of which are determined by the anodizingpotential between the electrolyte and the anodic aluminum article. Thus,it has been found that using a 15% sulfuric acid bath, the pore diameteris approximately Angstrom units and the wall thickness of each pore isapproximately 8 Angstrom units per volt. It has been found that thereare approximately 400x 10 pores per square inch of anodic surfaces.Additional measurements of the dimensions of the pores in the anodicfilm have indicated that the volume occupied by the pores isapproximately 5-15 of the entire volume of the film.

In order to reduce the porosity of anodic films there has long been useda sealing step carried out commercially by treatment of the anodizedaluminum with hot water at a temperature between about F. and theboiling point. This sealing step is essentially a chemical hydration ofthe anodically formed anhydrous oxide film. If the anodically formedfilm is subject to prolonged atmospheric exposure or to contact with amoist environment at a temperature below about 180 F. hydrationgradually takes place with the formation of aluminum oxide trihydrate.This form of oxide has been found by experience to be less desirablefrom a protective standpoint than that obtained by sealing with hotwater, whereby aluminum oxide monohydrate is formed. The mechanism ofmonohydration has been described in Surface Treatment and Finishing ofAluminum and Its Alloys, by S. Ernick and R. Pinner, pages 354-369.During monohydration of the anodic oxide, a 1020% film volume increaseis achieved, and it is believed that the pores are substantially closedby. this hydration step. Experience has proved, however, that simple hotwater sealing fails to give adequate protection to the coating againstcorrosion. Accordingly, attempts have been made in the prior art toimprove corrosion resistance of the anodic coatings by incorporating inthe sealing bath various inorganic and organic chemicals, such asdichromates, nickel acetate, chlorinated paraffin, molybdates, and thelike. Many of these chemicals are disadvantageous in that they result inimparting a dark shade to the aluminum oxide, thereby impairing itsusefulness in applications where the original bright color of thealuminum is to be preserved. Thus, While the dichromates and chromatesare effective anticorrosion agents, they must be applied inconcentrations Well above 0.1% and up to about 5% by weight to produceuseful results, and these concentrations result in excessive yellowtinting of the surface of the anodic coating. On the other hand, the useof concentrations of dichromates and chromates below about 0.1% byweight, while avoiding the tinting problem, does not result in effectivecorrosion protection. Accordingly, it has been proposed in US. Patent2,899,368, to employ molybdates of alkali metals as a sealing treatmentfor anodic coatings, in con centrations between 0.1% and 5%, andpreferably about 2%. But the treatment with molybdates alone, whileavoiding discoloration, still produces a final light grey color, takesas long as treatment with dichromate alone, and of itself produces nosubstantial improvement in corrosion resistance compared withdichromate.

Accordingly, it is an object of this invention to provide an improvedsealing and corrosion inhibiting treat ment for anodic coatings onaluminum and aluminum base alloys which will avoid discoloration of thecoating,

forms of corrosive attack.

taining not more than 200 p.p.m. of total solids.

reduce the time of treatment, and at the same time furnish virtuallycomplete protection against all common It is a further object of theinvention to provide sealing and corrosion inhibiting agents which willexert their action by adsorption on the anodic layers, but it is to beunderstood that applicant is not bound by any particular theory of themanner in which the novel agents of this invention produce their effect.

In accordance with one aspect of the present invention, it has beenfound that anodically formed aluminum oxide layers or films may besealed and protected against corrosion by treatment with a diluteaqueous solution of a water soluble basic organic nitrogen compoundwhich does not contain a polar substituent, such as, for example, ahydroxy group; or a water soluble salt thereof. Advantageously, there isemployed for this purpose a basic organic nitrogen compound containingone or more nitrogen atoms in a heterocyclic ring, and which does notcontain a polar substituent, such as, for example, a pyridine,quinoline, or quinazoline ring containing compound, or a water solublesalt thereof. Examples of heterocyclic nitrogen compounds which may beused include:

Nicotinamide Ethyl qinolinium iodide l-ethyl-2.G-dimethylquinoliniumiodide 6,8-dichloro-2.4-( 1H, 3H) -quinazolinedione.

There may also be employed water soluble carbocyclic compoundscontaining basic nitrogen in the form of a primary or unsubstitutedamino group, such as, for example derivatives of aniline. Examples ofsuch compounds include:

m-Tolylurea Ethyl p-aminobenzoate.

These heterocyclic and carbocyclic compounds may be incorporated in thesealing baths in amounts ranging from as low as about 0.001% by weightto the limit of their solubility, but preferably in an amount rangingfrom about 0.1% to about 1.0%. They may be employed singly, or one ormore of the compounds may be employed in conjunction. Their sealing andcorrosion inhibiting action is believed to be attributable to adsorptionon the anodic layer, accompanied by crosslinking effects, but applicantdoes not wish to be bound by any particular theory of action.

The organic nitrogen compounds will normally be applied to the coatingsin aqueous solution at a temperature between about 160 F. and theboiling point, preferably between 180" F. and the boiling point, and ata pH between about 5 and about 8. They should therefore be stable atthis pH range, and have a solubility in water greater than 1 millimoleper liter at 25 C. Treatment time is about 15 minutes, but this may bevaried according to materials and conditions.

Corrosion of anodic coatings is of two types: (1) pitting, characterizedby formation of small pits bordered by whitish areas, and (2) clouding,hazing or bloom, evidenced by staining in irregular areas.

Two standard corrosion testing procedures have been used to evaluate theperformance of the sealing and corrosion inhibiting compositions of thisinvention.

The test designated as the copper accelerated acetic acid salt spraytest (CASS) was used to evaluate the performance of variations in themethod of scaling in terms of resistance to accelerated corrosionenvironment. Briefly, the test employs a conventional salt spray cabinethaving an exposure chamber provided with plastic coated sample racksallowing placement of test samples between 15-30 from the vertical andparallel to the principal direction of flow of corrosive sprays. Thesalt-solution employed is prepared by dissolving :1 parts by weight ofsodium chloride in 95 parts of distilled water con- The pH of thissolution is adjusted to 3.2 by addition of acetic acid following which 1gram of cupric chloride is added for each gallon of the salt solution.The exposure chamber is maintained at a temperature of F. At theconclusion of the test, the exposed side of each panel is rinsed indistilled water and allowed to dry, permitting a qualitative orsemi-quantitative evaluation of corrosive attack to be made.

A second corrosion test is known as the Corrodkote test. This is carriedout in a humidity cabinet maintained at 98l00 F. with a relativehumidity of 99 100%. In this test a corrosive paste is applied to thesurface of the sample. The paste has the composition:

FeCl -6H O g 0.99 Cu(NO -3H O g 0.21 NH Cl g 6.00 Kaolin (Florida,air-floated) g 180 Water (distilled or deionized) ml 300 After exposure,the samples are evaluated for corrosion.

The novel method of the present invention employing organic nitrogencompounds as additions to aqueous sealing baths and the improvedcorrosion resistance obtained is illustrated in Examples 1 and 2, but itis to be understood that these examples are not to be regarded aslimiting.

In accordance with another aspect of this invention, the aforementionedorganic nitrogen compounds may be employed in conjunction with one ormore water soluble salts of metals of Group VI of the Periodic System,such metals including, for example, chromium, molybdenum, and tungsten.The chromium salt is preferably one in which the chromium is hexavalent,such as the chromates or dichromates. Ordinarily the alkali metal salts,including ammonium, sodium, and potassium chromates or d-ichromates willbe employed. The molybdenum and tungsten salts may be employed in theform of alkali metal salts, such as sodium or ammonium or potassiummolybdate, or tungstate.

The organic nitrogen compounds when added to the sealing bath act toretard the hazing type of corrosion, and are superior in this respect toeither the-dichromates or the molybdates. The latter are effectivemainly in retarding the pitting type of corrosion.

However, when an organic nitrogen compound such as nicotinamide, and,for example, a dichromate, are employed in conjunction as bathadditives, it is found that whereas the nicotinamide alone in a giventest time of say 24 hours produces a satisfactory degree of corrosionprotection, the combination of the organic compound with the dichromateimparts complete freedom from both pitting and clouding types ofcorrosion in the same test time. A similar effect is obtained withcombinations of organic nitrogen compound and a molybdate. These effectsare illustrated in Examples 2 and 3 below.

An even more complete corrosion protection is obtained when using thecombination of organic nitrogen compound, dichromate and molybdate,illustrated in EX- ample 4.

The remarkable degree of corrosion protection thus obtained appears torepresent more than the additive effect of the individual bathingredients, and to be indicative of a po-tentiating or synergisticeffect. Moreover, the combination of compounds makes possible a drasticreduction in treating time, ranging from as little as /2 minute up to 15minutes, as compared with normal sealing times of about 30 minutes asemployed in this industry, with resultant'economies.

The pH of the treating baths with the combination of organic andinorganic chemicals may range from about 5 to 8 in value, but a pH rangeof 6.0 to 6.5 is preferred. Bath temperatures range from F., andpreferably F. to the boiling point of the bath. The di-chromate orchromate concentration will generally be below about with as little as0.005% present.

5 0.1% by weight, down to about 0.005% by weight, thereby avoidingdanger of tinting by the chromium salt. Whereas the effective lowerlimit of concentration for the dichromate would ordinarily be about0.01%, the synergistic effect of the organic nitrogen compound makes itpossible to obtain an equivalent protection against pitting with the useof only 0.005% dichromate. However, the preferred concentration ofchromium salt is about 0.01%. The concentration of molybdate ortungstate may range from about 0.1% upward, but 0.1% is generallypreferred.

The combination of the organic nitrogen compound and at least one saltof chromium or molybdenum makes possible not only complete corrosionprotection, but satisfactory sealing in as little as /2 to 1 minute, andexcellent sealing in from 10 to 15 minutes.

'In accordance with another aspect of the invention, anodically formedaluminum oxide layers are treated with sealing baths containingcombinations of two or more water soluble salts of metals of Group VI ofthe Periodic System. Advantageously, there are employed combinationsofsalts of hexavalent chromium, such as the previously mentionedchromates or dichromates, together with water soluble molybdates. Herealso a potentiating or synergistic action exists between the dichromateon one hand and the molybdate on the other. It is believed that actionof the dichromate and molybdaate is one of deposition or ofabsorption-adsorption on the surface of the anodic-ally formed aluminumoxide layer of these ions in the form of double salts. Such a mechanismof deposition would necessarily limit the amount of surface availablefor molecular deposition. If either the dichromate or the molybdateagents are applied individually the surface will be saturated withrespect to the particular agent. Moreover, if used in conjunction, itwould be expected that the total deposition would be proportionate tothe concentrations of the individual agents, and that no improvementcould be expected beyond the additive effect of the two components.However, in accordance with the invention, it has been found that, byreason of the aforesaid deposition mechanism, the concentration of thedichromate, when used together with the molybdate, can "be safelyincreased well beyond the point which would result in yellowing of thecoating when such concentration of dichromate is used by itself.Conversely, the molybdate-dichromate combination makes possible areduction in the concentration of dichromate, not only below what wouldordinarily be its effective lower limit for producing corrosionresistance when employed alone, namely about 0.01% by weight, butenables the dichromate to attain equivalent corrosion resistant efficacyThe dichromates used must be compatible in solution with the molybdates,and both must be stable at the temperature employed. The dichromateconcentration employed will generally be below about 0.1% by weight,thereby avoiding the danger of tinting. The presence of the molybdatepermits this upper limit to be lowered to about 0.05% while stillproviding equivalent results. In general, however, it is preferred touse a chromate or dichromate concentration of about 0.01%. The molybdateconcentration may range from about 0.1% to about 1.0%, but 0.1% ispreferred.

The treating bath containing the combinations of chromium and molybdenumsalts will have a pH in the range of about 5.0 to 8.0, and preferablybetween about 6.0 and 6.5, and the bath temperature will range from 160F, and preferably 180 F. to the boiling point. Satisfactory sealing canbe obtained in as little as /2--l minute treatment time, and excellentsealing in-l-15 minutes. This aspect of the invention is illustrated inExamples 5, 6, 7, and 8 below.

It is to be understood that the novel method of this invention isapplicable not only to bright anodized articles. The method can also beapplied to matte sur- 6 faced anodized products such as those used forbuilding construction. The method can also be applied to the protectionof anodic films such as those produced by hard coat anodizing with theelectrolyte cooled to 40 C., forming an abrasion resistant coating.Coatings of this type are not usually sealed because they would softenin the hot water used for sealing. However, if a comparatively thin hardcoat is applied, say 0.3 mil thickness, then this type of coating can beeffectively sealed and protected against corrosion with a bath of anorganic nitrogen compound and a molybdate.

The following examples illustrate the invention, but are not to beregarded as limiting.

Example 1 A series of aluminum panels of alloy 5557 (the AluminumAssociation designation for a high purity magnesium-containing alloy)were prepared by mechanical buffing, chemical polishing in aphosphoric-nitric acid solution, anodizing in 10% sulfuric acid at F.using a current density of 15 amps/ft. to a film thickness of 0.3 mil.The surfaces were then sealed in a solution containing 0.1% nicotinamidedissolved in distilled water with a pH of 6.5. This solution wasmaintained at a temperature of 208 F. and the sealing continued for 15minutes. A group of samples of the same alloy was subjected to theentire finishing sequence with the exception of the sealing step. Thisgroup was sealed in a conventional distilled water uninhibited bath atthe same operating conditions. Subsequent to the finishing, the sampleswere subjected to a 20 hour exposure to the Corrodkote test andevaluated for evidence of corrosion. The

samples sealed in water only had developed a substantial amount ofhazing or bloom accompanied by the formation of numerous pittingextending through the oxide down into the metal. The samples sealed innicotinamide solution developed only a very slight amount of hazewithout any pitting.

' Example 2 Two groups of 565-7 alloy samples (Aluminum Associationdesignation for an aluminum alloy containing magnesium but of higherpurity than the 5557) were prepared by the finishing technique ofExample 1 with one group sealed in water. tion containing 0. 1% ethylp-aminobenzoate. The additions of this compound to the sealing bathprovided an increase in corrosion resistance at least comparable to thatobtained on the nicotinamide additions only as evidenced by bothCorrodkote and CASS accelerated exposure data.

Example 3 'Two additional groups of sample panels were prepared asindicated under Example 1 using the same alloy. In this series of tests,again one group was sealed in distilled .water only and the group to betested sealed in a solution containing 0.01% sodium dichromate and 1%nicotinamide at the previously mentioned operating conditions. Thisgroup was again subjected to the aforementioned Corrodkote test andevaluated. The water sealed samples reproduced the previously describeddegree haze of the film accompanied by pitting type corrosion. Thepanels sealed in nicotinamide-dichromate were free of pits and did notdevelop any clouding of the film.

Example 4 Twvo groups of samples of the previously mentioned alloy wereprepared as outlined in Example 1. One of the groups again served as areference by consisting of water sealed samples. The test groupconsisted of panels sealed in a solution containing 1% nicotinamide,.01% sodium dichromate, 0.1% sodium molybdate at the previouslymentioned operating conditions. At the conclusion of a 20 hourCorrodkote test, the sample sealed The other group .was sealed in asolu- Example 5 A series of aluminum panels, .04" gauge, was preparedusing the Aluminum Association alloy designated 5557. This alloycontains 99.8% pure aluminum with approximately .6% magnesium, .25%manganese and .06% copper added. To maintain a statisticallyreproducibility exceeding 95%, each group of samples was prepared innumbers of 11 each. All of the test panels were finished vby acommercially practiced sequence consisting of mechanical buffing,chemical polishing in a phos phoric-nitric acid type solution followedby anodizing in sulfuric acid using a current density of 12 ampsl/sq.ft. to an oxide film thickness of 0.3 mil. At this point, one group of11 samples was sealed by the conventional water sealing techniqueinvolving a 15-minute immersion in a distilled water bath maintainedbetween 210-212 The second group of 11 samples was sealed in a distilledWater bath containing additions of 0.01% sodium dichromate and 0.1%sodium molybdate at a temperature of 2.10-212 F. Both baths had anelectrometric pH of 6.0-6.5. The total group of v22 test panels weresubjected to the CASS test by randomizing the panels in the exposurecabinet. In periods of two hours each, the samples were removed andvisually examined for corrosive action. In this test, corrosive actionconstituting a failure consisted of the initial appearance of pittingtype corrosion or clouding of the anodic film. In each case, the time tothe nearest two hours was noted for initiation of this type of failure.The entire group of panels sealed in distilled water only had sustainedan initiation of corrosion after 6-8 hours exposure. Of the other groupof panels, those sealed in dichromate-molybdate, nine of the panels hadnot yet developed corrosion after 16'hours,

with several of the panels requiring up to 24 hours exposure beforefailing. From this test, it is apparent that the additions ofdichromate-molybdate enhance the corrosion resistance of anodizedaluminum by a factor of two.

Example 6 A second test consisting of preparing samples as outlinedunder Example 5 and exposing the entire randomized group to 20continuous hours in CASS. At the conclusion of the test, the sampleswere examined for corrosion. The group of panels sealed in distilledwater only had developed a substantial amount of surface hazing andcloudy areas. The dichromate-molybdate group had sustained only slighthazing.

Example 7 In order to study the comparative effect of individualadditions of sodium dichromate and sodium molybdate to the water sealingbath, a series of tests was carried out, in which the samples wereexposed for 20 continuous hours in the CASS test, and then the pitingrating range was measured in accordance with ASTM procedure, employing ascale in which 10 indicates no corrosive attack and rating is given whenthe entire surface is corroded. The general procedure outline inExamples 4 and was used. A 0.3 mil anodic coating was first applied asindicated in Example 1.

(a) Water seal only: a group of samples sealed in distilled Water onlyand exposed for 20 continuous hours in the CASS test showed a pittingrating range of 1-3.

(b) A sample group sealed with an aqueous 0.01% sodium dichromatedihydrate solution, after 20 hours exposure showed a pitting ratingrange of 2-5.

(c) A sample group sealed with an aqueous 0.1% sodium molybdatedihydrate solution for 20 hours, showed a pitting rating range of 2-5.

((1) A sample group sealed with an aqueous solution of 0.01% sodiumdichromate dihydrate and 0.1% sodium molybdate dihydrate, for 20 hours,showed a pitting rating range of 5-9. From these performances, it isclearly demonstrated that the combination of dichromate and molybdateproduced a synergistic corrosion inhibiting effect beyond that of thesecompounds taken individually.

Example 8 In order to evaluate the optimum concentration of the sodiumdichromate without rendering excessive yellow tinting to the surface,four series of samples in multiples of 11 were prepared as described inExample 5. Successively larger sodium dichromate concentrations were employed in the sealing bath for each group ranging from 0.001, 0.01, 0.10and 1.0. All panels were exposed in the CASS test for 24 hours andexamined every two hours. A concentration of 0.001% resulted in failurein 4-6 hours as evidenced by the presence of a limited number of pitsaccompanied by clouding of the film. The .O1% concentration failed inthe 13-16 hours which was also the case for the 1%. The 1% concentrationrendered a slight yellow color to the anodic film which would make itcommercially unacceptable for decorative applications. The testdescribed under Example 8 illustrates what constitutes one of the mainproblems in obtaining increased corrosion resistance of bright anodizedaluminum. This consists of retarding the formation of a clouding orhazing of the aluminum surface traceable to attack or deposit in or onthe surface of the anodic oxide film. In this respect, the apparentoptimum concentration for sodium dichromate consists of 0.01%. Anotheraspect of corrosive action consists of the development of pits in theanodic film principally caused by galvanic type corrosion. In thisrespect, increased concentrations thus provide improved resistance with0.01% giving for its concentration level highest equivalent resistance.

Example 9 The following results using the Corrodkote testing techniquewere obtained on samples prepared in accordance with the sequenceoutlined in Example 1 with one group sealed in a conventional water sealand the other in the dichromate-molybdate version of the sealing bath.The group sealed in water only exhibited a very small amount of pittingbut the surface was completely covered with a light diffusing appearinghaze or bloom. In contrast, the group sealed in dichromatemolybdateshowed no evidence of pitting with the haze being noticeably reduced. Asa practical form of evaluating these surfaces, representative samplesfrom each of these groups were subjected to a wax cleaning step usingautomotive type commercial compounds. The water sealed samples showedvirtually no improvement resulting from this treatment. In contrast,those sealed in dichromate-molybdate exhibited as a result of thetreatment a surface closely resembling the initial appearance.

While it is ordinarily advantageous to accomplish sealing of the anodiccoating in the solution used for corrosion protection treatment, aspreviously described, the solution also may be applied at a lowertemperature than that required for sealing purposes, provided thesolution temperature is above its freezing point; and sealing of thesolution-treated coating may be accomplished subsequently in anyconvenient manner. In addition, relatively thick hard-coat anodizedaluminum is preferably treated at a solution temperature low enough toavoid crazing or cracking of the anodic coating. An example of thelatter practice is the following:

Example 10 Panels of aluminum alloy 5052 were prepared by degreasing,alkaline etching and nitric acid desmu-tting, using water rinses wherenecessary and conventional treating solutions. The panels weresubsequently hard anodized in a 20% by volume sulfuric acid electrolyteat 30-32 F.

using a current density of 24 amps./ft. An oxide film of 11.2 mils wasproduced.

Half of the panels were left untreated after anodizing, rinsing anddrying. The others were immersed in a solution of Percent by weightSodium molybdate 0.1 Sodium dichromate 0.01 Nicotinamide 0.1

at pH 6.5 and temperature of 120-125 F. for 15 minutes, rinsed andallowed to dry.

' The panels treated in the dichromate-molybdate-nicotinamide solutionat 120-l25 F. did not show the crazing or appearance of numerous finecracks usually evident in hard anodic films when treated in hot aqueoussolutions.

The two groups of samples were subjected to exposure for 16 hours incopper chloride-acetic acid salt spray (CASS). The panels left untreatedafter anodizing showed considerable corrosion, pitting and staining andwere rated 1-2 on a scale'where denotes complete coverage of corrosionand 10 denotes no corroded areas. The panels treated in theaforementioned solutionhad substantially less corrosion and were rated6-7.

What is claimed is:

11. Method for the treatment of an anodically formed coating on aluminumand aluminum base alloys for seal- I ing and inhibiting corrosionthereof, which comprises applying to said coating a dilute aqueoussolution of a water soluble basic organic nitrogen compound which doesnot contain a polar substituent, selected from the group consisting ofheterocyclic nitrogen compounds containing at least one basic nitrogenin a heterocyclic ring selected from the group consisting of pyridine,quinoline and quinazoline rings, mononuclear carbocyclic nitrogencompounds containing an unsubstituted amino group, and the water solublesalts thereof, at a temperature between about 160 F. and the boilingpoint of said solution, and at a pH between about and about 8.

2. The method for the treatment of an anodically formed coating onaluminum and aluminum base alloys (for sealing and inhibiting corrosionthereof, which comprises applying to said coating a dilute aqueoussolution of a water soluble basic organic nitrogen compound which doesnot contain a polar substituent, selected from the group consisting ofheterocyclic nitrogen compounds containing at least one basic nitrogenin a heterocyclic ring selected from the group consisting of pyridine,quinplying to said coating a dilute aqueous solution of a water solublebasic organic nitrogen compound which does not contain a polarsubstituent selected from the group consisting of heterocyclic nitrogencompounds containing at least one basic nitrogen in a heterocyclic ringselected from the group consisting of pyridine, quinoline andquinazoline rings, mononuclear carbocyclic nitrogen compounds containingan unsubstituted amino group, and the water soluble salts thereof, awater soluble hexavalent chromium salt, and a water soluble molybdate,at a temperature between about 160 F. and the boiling point of saidsolution, and ate. pH between about 5 and about 8.

4. Method for the treatment of an anodically formed coating on aluminumand aluminum base alloys for sealing and inhibiting corrosion thereof,which comprises applying to said coating a dilute aqueous solutioncontaining about 1% by weight of nicotinamide, about 0.01% by weight ofsodium dichromate, and about 0.1% by weight of sodium molybdate.

5. Aluminum and aluminum base alloys having an anodically formedcoat-ing thereon sealed and inhibited against corrosion by adsorption ofa water soluble basic organic nitrogen compound which does not contain apolar substituent selected from the group consisting of heterocyclicnitrogen compounds containing at least one basic nitrogen in aheterocyclic ring selected from the group consisting of pyridine,quinoline and quinazoline rings, mononuclear carbocyclic nitrogencompounds containing an unsubstituted amino group, and the water solublesalts thereof.

6. Aluminum and aluminum base alloys having an anodically formed coatingthereon sealed and inhibited against corrosion by adsorption of a watersoluble basic organic nitrogen compound which does not contain a polarsubstituent, selected from the group consisting of heterocyclic nitrogencompounds containing at least one group consisting of pyridine,quinoline and quinazoline rings, mononuclear carbocyclic nitrogencompounds containing an unsubstituted amino group, and the water solublesalts thereof, a hexavalent chromium salt, and a molybdate.

8. Method for the treatment of an anodically formed coating on aluminumand aluminum base alloys for inhibiting corrosion thereof, whichcomprises applying to said coating a dilute aqueous solution of a watersoluble basic organic nitrogen compound which does not contain a polarsubstituent, selected from the group consisting of heterocyclic nitrogencompounds containing at least one basic nitrogen in a heterocyclic ringselected from the group consisting of pyridine, quinoline andquinazoline rings, mononuclear carbocyclic nitrogen compoundscontraining an unsubstituted amino group, and the water soluble saltsthereof, at a pH between about 5 and about'8.

9. Method for the treatment of an anodically formed coating on aluminumand aluminum base alloys for inhibiting corrosion thereof, whichcomprises applying to said coating a dilute aqueous solution of a watersoluble basic organic nitrogen compound which does not contain a polarsubstituent, selected from the group consisting of heterocyclic nitrogencompounds containing at least one basic nitro'gen in .a heterocyclicring selected from the group consisting of pyridine, quinoline andquinazoline rings, mononuclear carbocyclic nitrogen compounds containingan unsubstituted amino group, and the water soluble salts thereof, andat least one water soluble salt of a metal of Group VI of the PeriodicSys tem, at a pH between about 5 and about 8.

10. Method for the treatment of an anodically formed coating on aluminumand aluminum base alloys for inhibiting corrosion thereof, whichcomprises applying to said coating a dilute aqueous solution of watersoluble basic organic nitrogen compound which does not contain a polarsubstituent, selected from the group consisting of heterocyclic nitrogencompounds containing at least one basic nitrogen in a heterocyclic ringselected from the group consisting of pyridine, quinoline andquinazoline rings, mononuclear carbocyclic nitrogen compounds containingan unsubstituted amino group, and the water solu- 1.1 ble salts thereof,a water soluble hexavalent chromium salt, a water soluble molybdate, ata pH between about and about 8.

'11. Method for the treatment of an anodically formed coating onaluminum and aluminum base alloys, which comprises applying to saidcoating a dilute aqueous solution of a water soluble basic organicnitrogen compound which does not contain a polar substituent, selectedfrom the group consisting of heterocyclic nitrogen compounds containingat least one basic nitrogen in a heterocyclic ring selected from thegroup consisting'of pyridine, quinoline and quinazoline rings,mononuclear carbocyclic nitrogen compounds containing an unsubstitutedamino group, and the water soluble salts thereof, at a pH between about5 and about 8, and thereafter sealing said coating in an aqueous mediumat a temperature of at least about 160 F. to effect hydration of thecoating.

12. Method for the treatment of an anodically formed coating on aluminumand aluminum base alloys, which comprises applying to said coating adilute aqueous solution of a water soluble basic organic nitrogencompound which does not contain a polar substituent, selected from thegroup consisting of heterocyclic nitrogen compounds containing at leastone basic nitrogen in a heterocyclic ring selected from the groupconsisting of pyridine, quinoline and quinazoline rings, mononuclearcarbocyclic nitrogen compounds containing an unsubstituted amino group,and the water soluble salts thereof, and at least one water soluble saltof a metal of Group VI of the Periodic System, at a pH between about 5and about 8, and thereafter sealing said coating in an aqueous medium ata temperature of at least about 160 F. to affect hydration of thecoating.

13. Method for the treatment of an anodically formed coating on aluminumand aluminum base alloys, which comprises applying to said coating adilute aqueous solution of a water soluble basic organic nitrogencompound which does not contain a polar substituent, selected from thegroup consisting of heterocyclic nitrogen compounds containing at leastone basic nitrogen in a heterocyclic ring selected from the groupconsisting of pyridine, quinoline and quinazoline rings, mononuclearcarbocyclic nitrogen compounds containing an unsubstituted amino group,and the Water soluble salts thereof, a water soluble hexavalent chromiumsalt, and a water soluble molybdate, at a pH between about 5 and about8, and thereafter sealing said coating in an aqueous medium at atemperature of at least about 160 F. to affect hydration of the coating.

14. Method for the treatment of an anodically formed coating on aluminumand aluminum base alloys, which comprises applying to said coating adilute aqueous solution containing about 1% by weight of nicotinamide,about 0.01% by weight of sodium dichromate, and about 0.1% by weight ofsodium molybdate, and thereafter sealing said coating in an aqueousmedium at a temperature of at least about 160 F. to affect hydration ofthe coating.

15. Method for the treatment of hard-coat anodized aluminum and aluminumbase alloys for inhibiting corrosion thereof, which comprises applyingto said coating a dilute aqueous solution of a water soluble basicorganic nitrogen compound which does not contain a polar substituentselected from the group consisting of heterocyclic nitrogen compoundscontaining at least one basic nitrogen in a heterocyclic ring selectedfrom the group con 12 sisting of pyridine, quinoline and quinazolinerings, mononuclear carbocyclic nitrogen compounds containing anunsubstituted amino group, and the water soluble salts thereof, at asolution temperature above ambient temperature but below the temperatureat which crazing of said coating may occur, and at a pH between about 5and about 8.

16. Method for the treatment of hard-coat anodized aluminum and aluminumbase alloys for inhibiting corrosion thereof, which comprises applyingto said coating a dilute aqueous solution of a water soluble basicorganic nitrogen compound which does not contain a polar substituentselected from the group consisting of heterocyclic nitrogen compoundscontaining at least one basic nitrogen in a heterocyclic ring selectedfrom .the group consisting of pyridine, quinoline and quinazoline rings,mononuclear carbocyclic nitrogen compounds containing an unsubstitutedamino group, and the water soluble salts thereof, and at least one watersoluble salt of a metal of Group VI of the Periodic System, at asolution temperature above ambient temperature but below the temperatureat which crazing of said coating may occur, and at a pH between about 5and about 8.

117. Method for the treatment of hard-coat anodized aluminum andaluminum base alloys for inhibiting corrosion thereof, which comprisesapplying to said coating a dilute aqueous solution of a water solublebasic organic nitrogen compound which does not contain a polarsubstituent selected from the group consisting of heterocyclic nitrogencompounds containing at least one basic nitrogen in a heterocyclic ringselected from the group consisting of pyridine, quinoline andquinazoline rings, mononuclear carbocyclic nitrogen compounds containingan unsubstituted amino group, and the water soluble salts :thereof, awater soluble hexavalent chromium salt, and a water soluble molybdate,at a solution temperature above ambient temperature but below .thetemperature at which crazing of said coating may occur, and at a pHbetween about 5 and about 8.

18. Method for the treatment of hard-coat anodized aluminum and aluminumbase alloys for inhibiting corrosion thereof, which com-prises applyingto said coating a dilute aqueous solution containing about 1% by weightof nicotina-m-ide, about 0.01% by weight of sodium dichromate, and about0.1% by weight of soditun molybdate, at a solution temperature aboveambient temperature but below the temperature at which crazing of saidcoating may occur, and at a pH between about 5 and about 8.

References Cited by the Examiner UNITED STATES PATENTS 2,132,619

525,734 9/1940 Great Britain.

RICHARD D. NEVIUS, Primary Examiner.

R. S. KENDALL, Assistant Examiner.

1. METHOD FOR TEH TREATMENT OF AN ANODICALLY FORMED COATING ON ALUMINUMAND ALUMINUM BASE ALLOYS FOR SEALING AND INHIBITING CORROSION THEREOF,WHICH COMPRISES APPLYING TO SAID COATING A DILUTE AQUEOUS SOLUTION OF AWATER SOLUBLE BASIC ORGANIC NITROGEN COMPOUND WHICH DOES NOT CONTAIN APOLAR SUBSTITUENT, SELECTED FROM THE GROUP CONSISTING OF HETEROCYCLICNITROGEN COMPOUNDS CONTAINING AT LEAST ONE BASIC NITROGEN IN AHETEROCYCLIC RING SELECTED FROM THE GROUP CONSISTING OF PYRIDINE,QUINOLINE AND QUINAZOLINE RINGS, MONONUCLEAR CARBOCYCLIC NITROGENCOMPOUNDS CONTAINING AN UNSUBSTITUTED AMINO GROUP, AND THE WATER SOLUBLESALTS THEREOF, AT A TEMPERATURE BETWEEN ABOUT 160*F. AND THE BOILINGPOINT OF SAID SOLUTION, AND AT A PH BETWEEN ABOUT 5 AND ABOUT 8.